1. Field of the Invention
The present invention relates to a process for measuring transmission factors and more particularly to a process for measuring atmospheric transmission factors and a device employing this process.
2. Description of the Prior Art
As is disclosed in the monograph No. 76 of the French National Meteorological Service published in August 1970, the transmission factor T is defined for a light beam as the ratio of the light fluxes at the beginning and end of an optical path of specified length in a dim atmosphere. This factor is sometimes known as the transmittance .tau. for a defined path of predetermined length. Devices for measuring the transmission factor of the atmosphere are generally known as transmissometers. The basic principle of a transmissometer consists in measuring the average transmissivity of the atmosphere along a predetermined path. In practice this involves arranging a projector with a constant flux having perfectly defined optical and mechanical features so that it illuminates a receiver which also has a well defined geometry and which is situated at a distance D from the projector. The current or output voltage of the receiver enables the transmittance of the density of the interposed atmosphere and thus its average transmissivity to be determined, the transmissivity being the transmission factor per unit of length.
A device of this type can also be graduated directly in terms of meteorological visibility by using formulae linking these quantities to transmission factors. The visibility is the maximum distance at which a reference can be identified by an average observer.
A distinction is drawn between the daytime meteorological visibility and nighttime meteorological visibility. The daytime meteorological visibility or visibility by contrast is defined as the greatest distance at which a dark object of suitable dimensions situated close to the earth can be seen and identified when it is observed against a background of mist or sky on a generally horizontal plane. The nighttime meteorological visibility which is a physical feature of the atmosphere is determined from observation of specific light sources as being the distance at which the coefficient of extinction reaches a certain value. According to the Koschmieder theory the relationship between the transmission factor T and the visibility by contrast V for a path having the length D is the following: ##EQU1## where .epsilon. is the coefficient of extinction for which the value ##EQU2## is generally selected for meteorological purposes and the value ##EQU3## for aeronautical purposes. According to the Allard theory the relationship between the transmission factor and the nighttime visibility V' (or the visibility of a light source) is the following: ##EQU4## where I is the intensity of the light source and E.sub.t a threshold of visual illumination, this value E.sub.t being based on the ambient light according to the defined norms. This visibility measurement is especially useful for airports, highways or motorways.
If the principle of measuring transmittance indicated above were applied directly to a transmissometer, gross errors would result. In the first place, the flux emitted is not known precisely. It is a function of the dispersion of the active emission elements and the time and temperature. Secondly, the flux received is a function of the gain of the photoreceiving cell, and it also depends on the features of these cells and on their variation with time and temperature. In addition, the flux received has also been contributed to by the ambient light which tends to falsify any measurement.
Various types of transmissometers have been proposed in the prior art to obviate these disadvantages. Firstly there are transmissometers in which a calibration of the emitting lamp is first obtained by using a reference path between this lamp and the photoreceiving cell and by effecting the switching operation between these two paths by mechanical means. There is no need to enumerate at this point the disadvantages of these mechanical devices which result essentially from the permanent maintenance which they require. Static devices were then developed. All these devices are based on the idea that it is necessary to free the emitters and receivers from variations and the light fluxes emitted are modulated to separate them from the ambient light fluxes. For example, it is possible to cite a device comprising a double emitter- receiver unit designed to correct errors due to the aberrations of these various elements. One obvious disadvantage of a device of this kind is the complexity of its elements. There are other devices where there is an optical coupling between the emitter and the remote receiver. This optical coupling involving constant losses is constituted, for example, by means of optical fibers. One disadvantage of this type of device consists in the very need for an optical coupling which requires very long optical fibers when there is a fair distance between the receiver and the emitter. Very long optical fibers of this type are in fact expensive and their length is by necessity dictated by the limits of current technology.